Maus' summary of Computer Games article published approx. 7/01
4) Crops. Q: One of the most frustrating things in a game is to develop a dependency on a single cash crop, only to notice later that the land devoted to the farms has turned orange, indicating that it is unsuitable for that crop. Is crop rotation necessary, or is the change purely climatic? Can you anticipate changes by using the info overlays for soil and rain?
A: Rotating crops has no impact. It's purely climate related, based on rainfall. The initial values for crop suitability are are based on the game running an internal weather model for 80 years at game start, and averaging the results. Of course, it's a long-term average, and there are short-term fluctuations. Steinmeyer: "I usually just accept that there will be good and bad harvests, based on the rainfall... I don't typically adjust my crop patterns too much after I've built a farm."
However, a couple of posts in this thread seem to indicate that the radiated pollution effects of buildings and the impact of units moving (perhaps unit radiated pollution) have a cumulative, negative effect on soil quality. I know of no careful study and report of that issue. None-the-less, crop rotation is not particularly useful just for itself. It won't hurt for other planned changes of land use. The farm boundaries are five tiles from the building.
Remember, T3 greatly elaborated the farming model.